(iv) If \(A\)  is the generator of a strongly continuous group \(\lbrace U_A(t)\rbrace _{t \in {\mathbb {R}}}\) 
of contractions \(\Vert U_A(t)\Vert  \le 1\) , then \(\pm A \in {G}(1,0)\) , and
\(A^2 \in {H}_{c}(\pi /2,0)\) , [1]}, Corollary II.4.9.

